Power semiconductor devices such as switching devices and diodes are used for circuits of switching power supplies and inverters. These power semiconductor devices are required to have a high breakdown voltage and low on resistance. Then, there is a tradeoff relationship between the breakdown voltage and the on resistance determined by a semiconductor material used for the devises.
With the progress of technological development heretofore, the low on resistance of silicon as a main device material has been realized close to limitations thereof for power semiconductor devices. To further reduce the on resistance, it is necessary to change the semiconductor material. By using a nitride semiconductor such as GaN or AlGaN or a wide band gap semiconductor such as silicon carbide (SiC) as the switching device material, the tradeoff relationship determined by the semiconductor material can be improved and dramatically lower on resistance can be expected.
Among devices using a nitride semiconductor such as GaN or AlGaN, devices that can easily achieve lower on resistance include, for example, a heterojunction field effect transistor (HFET) using AlGaN/GaN hetero structure. The HFET realizes low on resistance by high mobility of hetero interface channels and a high electron concentration generated by polarization. Accordingly, low on resistance is achieved even if the chip area of a device is small.
However, it is difficult to reduce interface resistance by forming an alloy between AlGaN or GaN and a metallic material of an electrode. This is because the diffusion coefficient of metal in AlGaN or GaN is small. Therefore, a problem is remained that contact resistance of a source electrode and a drain electrode cannot be reduced, leading to higher on resistance.